1. Field of Use
This invention relates generally to proportional flow control valves for operating double-acting cylinders, fluid motors and similar devices. In particular, it relates to proportional flow control valves comprising an electro-hydraulic proportional servo actuator or servo means for operating a directional control valve to control fluid flow to the cylinder or other aforementioned device.
2. Description of the Prior Art
The prior art discloses a wide variety of electro-hydraulic proportional servo actuator operated proportional flow control valves and U.S. Pat. Nos. 2,771,062 and 3,000,363 exemplify this. In some such prior art valves the servo actuator or servo means includes a pilot valve operable by a proportional solenoid or similar electro-magnetic device to effect axial movement of the valve spool of a directional control valve. Such prior art arrangements depend on the principle of applying continuous differential fluid pressures on pistons connected to the valve spool. Therefore, maintaining the control valve spool in null position depends upon precise control of differential pressures by the pilot valve over long periods of time. However, passages and orifices in the pilot valves are of relatively small diameter and can be easily partially clogged or blocked by fine unfiltered contaminants in the fluid. As a result, the partially blocked pilot valve orifices change the differential pressures acting on the piston areas and the control valve spool shifts from the null position thereby resulting in undesirable operation of the valve and the cylinder or other device being controlled. Filtering of the fluid is not an entirely satisfactory solution to this problem and contaminants and silt deposits can build up over a period of time, necessitating valve disassembly, cleaning or valve replacement.